1. Field of the Invention
The present invention relates to wireless communications; more specifically, a method for providing transmit diversity.
2. Description of the Prior Art
Both transmit and receive diversity are used to counter channel fading. In the case of a receiver, diversity is provided by using two antennas that are spaced a sufficient distance apart so that only one antenna experiences a fading signal at one time. Similarly, transmit diversity is provided using two or more antennas that are spaced a sufficient distance apart so that a receiver is unlikely to have the signals from all of the antennas simultaneously fade.
FIG. 1 illustrates a prior art CDMA (Code Division Multiple Access) transmitter providing transmit diversity. Encoder 10 receives data to be transmitted and adds coding such as error correction and detecting coding. The data is then passed to interleaver 12 that reorders the data so that the loss of consecutive bits can be spread out in time when the bits are reordered at a receiver. The output of interleaver 12 is provided to demux 14 which splits the data into two parallel paths that are provided to multipliers 16 and 18. Multipliers 16 and 18 encode the data using orthogonal codes such as Walsh codes Wn1 and Wn2. It should be noted that by passing through demux 14 the data rate is decreased by one half. It should also be noted that one CDMA channel typically uses a single Walsh code such as Walsh code Wn. Since the data rate is decreased by one half, the Walsh code Wn may be broken into two longer orthogonal Walsh Codes Wn1 and Wn2. Equations 1 and 2 illustrate the relationship between longer Walsh Codes Wn1 and Wn2 and shorter Walsh Code Wn.
Wn1=[Wn, Wn]xe2x80x83xe2x80x83(Eq. 1)
Wn2=[Wn, xe2x88x92Wn]xe2x80x83xe2x80x83(Eq. 2)
An example of generating two longer Walsh Codes from a single Walsh code is illustrated by Equations 3, 4 and 5.
Wn=1 1 xe2x88x921 xe2x88x921xe2x80x83xe2x80x83(Eq. 3)
Wn1=1 1 xe2x88x921 xe2x88x921 1 1 xe2x88x921 xe2x88x921 xe2x80x83xe2x80x83(Eq. 4)
Wn2=1 1 xe2x88x921 xe2x88x921 xe2x88x921 xe2x88x921 1 1 xe2x80x83xe2x80x83(Eq. 5)
Equation 3 illustrates a simple four bit Walsh code and Equations 4 and 5 illustrate longer Walsh codes Wn1 and Wn2, respectively. It can be seen that Walsh code Wn1 is simply two repetitions of Walsh code Wn and that Walsh code Wn2 is Walsh code Wn followed by xe2x88x921 times Walsh code Wn.
Returning the FIG. 1, multipliers 20 and 22 apply a pseudo-random code to each of the data paths and then the data is passed to RF sections 24 and 26. The RF sections perform functions such as modulating a carrier signal having a carrier frequency f1 with the encoded data and providing sufficient amplification before transmission over antennas 28 and 30. It should be noted that the system of FIG. 1 provides transmit diversity by dividing the data into two paths that transmit at the same frequency over two antennas; however, the two paths maintain an orthogonal relationship by using different Walsh codes to encode the data.
FIG. 2 illustrates a second CDMA transmitter providing transmit diversity. As in FIG. 1, the data is processed by encoder 10 and interleaver 12 before being passed to a demux. Demux 40 divides the data into three parallel paths that are provided to multipliers 42, 44 and 46. Each of the multipliers encodes the data using Walsh code Wn. The data from multipliers 42, 44 and 46 is then passed to multipliers 48, 50 and 52, respectively, where the data is further encoded with a pseudo-random code. The data from multiplier 48 is provided to RF section 54 which modulates the data onto a carrier having frequency f1. The data from multiplier 50 is provided to RF section 56 which modulates the data onto carrier having frequency f2. The data from multiplier 52 is provided to RF section 58 which modulates the data onto a carrier having frequency f3. The output of the RF sections is provided to antennas 60, 62 and 64. In this case, transmit diversity is provided using three antennas where the orthogonality of the three channels is provided by the use of different carrier frequencies.
The present invention provides a transmitter for wireless communications that has multiple types of orthogonality to improve transmit diversity. Transmit diversity is improved by using both coding and carrier frequency orthogonality. Data to be transmitted is broken into four parallel channels. Two of the channels are transmitted on a first carrier signal and the other two channels are transmitted on a second carrier signal. Channels transmitted on the same carrier signal are provided with orthogonal codes so that they may be separated by a receiver. Channels transmitted on different carrier signals may be encoded with identical orthogonal codes. The modulated carrier signals are then transmitted using at least two antennas, where one antenna is used for each carrier. It should be noted it is also possible to transmit both carriers on each of the antennas.